Crystalline ZSM-5 and its conventional preparation using tetrapropylammonium cations as a structure directing agent are taught by U.S. Pat. No. 3,702,886 and U.S. Pat. No. Re. 29,948. Conventional ZSM-5 has a distinctive X-ray diffraction pattern which distinguishes it from other known crystalline materials and is a highly versatile catalyst useful in a variety of organic conversion reactions. The unique framework type of ZSM-5 has been assigned the framework type MFI by the Structure Commission of the International Zeolite Association (see “Atlas of Zeolite Framework Types” C. Baerlocher, L. B. McCusker, D. H. Ohlson, Sixth Revised Edition 2007). Examples of other zeolites that have the MFI framework type include: AZ-1, encilite, FZ-1, LZ-105, mutinaite, NU-4, NU-5, silicalite (silica form of ZSM-5), TS-1, TSZ, TSZ-III, TZ-01, USC-4, USI-108, ZBH, ZKQ-1B, ZMQ-TB.
In addition to tetrapropylammonium cations, a large number of other organic nitrogen compounds, including certain diquaternary ammonium compounds, have been shown to direct the synthesis of ZSM-5. For example, U.S. Pat. No. 4,585,638 discloses that the synthesis of ZSM-5 can be directed by the diquaternary cation (alkyl)3N+(CH2)6N+(alkyl)3, where the alkyl group is propyl or butyl.
For some acid-catalyzed reactions over zeolites, it is beneficial to reduce diffusion lengths of the reagent and/or product molecules by employing a zeolite with a reduced crystal size. Small crystals also have the benefit of providing high surface area.
An example of small crystal ZSM-5 is disclosed in U.S. Pat. No. 5,240,892, in which the ZSM-5 is in the form of platelets having first and second major dimensions of at least about 0.05 micron, preferably at least about 0.1 micron, and a minor third dimension of less than about 0.02 micron, preferably less than about 0.01 micron. The ZSM-5 has a mesitylene sorption capacity of at least 3.0 weight % and is produced using precipitated silica as the silica source either in the absence of an organic directing agent or using n-propylamine as the directing agent.
In addition, in “Molecular and Meso- and Macroscopic Properties of Hierarchical Nanocrystalline ZSM-5 Zeolite Prepared by Seed Silanization”, Chem. Mater. 21 (2009) 641-654, D. Serrano et al. report synthesizing ZSM-5 crystals as small as 5 to 10 nm using a dual template of tetrapropylammonium (TPA) ions and phenylaminopropyltrimethoxysilane. In this method, the silanizing agent is introduced after the synthesis gel is pre-heated for short periods of time before the onset of zeolite crystallization.
Ryoo and coworkers have reported in “Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived, catalysts”, Nature 461, 246-249 (10 Sep. 2009), the synthesis of a single unit cell-thick version of ZSM-5 by using a single templating agent composed of a 22-carbon atom alkyl chain and two quaternary ammonium groups separated by a methylene chain of 6 carbon atoms. Here the quaternary ammonium groups are located within the single-unit cell nanosheets, which are separated from one another by the long alkyl chains.
In “Zeolite Synthesis Using Flexible Diquaternary Alkylammonium Ions (CnH2n+1)2HN+(CH2)5N+H(CnH2n+1)2 with n=1-5 as Structure-Directing Agents”, Chem. Mater. 2005, 17, 477-486 (1 Jul. 2005), Han et al. report the use of Et6-diquat-5 to produce ZSM-57 and the use of Me4-diquat-5 to produce LEV and MCM-22.
Liao et al. report in “Synthesis, characterization of COK-5 with different Si/Al ratios and their catalytic properties for the tert-butylation of phenol”, Microporous and Mesoporous Materials, 124 (2009) 210-217, the synthesis of COK-5 with Et6-diquat-n (n=5, 6).
In “Diquaternary Ammonium Compounds in Zeolite Synthesis: Cyclic and Polycyclic N-Heterocycles Connected by Methylene Chains”, J. Am. Chem. Soc. (2009), 131, 1092-1100, Jackowski et al. disclose MFI synthesis using N-heterocycle structure directing agents.
Caullet et al. report in “Synthesis of Zeolites in the Presence of Diquaternary Alkylammonium Ions as Structure-Directing Agents”, Oil & Gas Science & Technology, Vol. 62 (2007), No. 6, pp. 819-825, the use of bis (n-trimethylaminoalkyl)ethers in the production of mainly ZSM-48 but also MFI type zeolite.
For several zeolites, and in particular those with the MFI structure, several morphologies are available with tunable crystal size for different applications. Examples of such morphologies are coffins, intergrown coffins, spheres and needles. In some cases however, it may be desirable to have the zeolite crystals with platelet morphology. Furthermore, for applications which can be limited by diffusion it is desirable to have small crystals. Crystal shape or morphology and size are thus important properties of molecular sieves.
The ability to control crystal shape and crystal size provides the ability to fine-tune a molecular sieve for specific applications in catalysis and molecular separations. Therefore, there is a need for a method to control the crystal size and morphology of MFI framework type molecular sieves, such as ZSM-5.
According to the present invention, using particular structure directing agents, it has now been found that new forms of crystalline molecular sieve with MFI framework type can be synthesized.